The present invention relates to a multi-band predistorter that permits amplification of all transmission signals of multiple frequency bands by a common amplifier.
One of known compensation method for nonlinear distortions by microwave power amplifiers is a predistortion method using a digital signal processing scheme (hereinafter referred to as a digital predistortion method) (for example, Japanese patent application laid open No. 2002-522989, hereinafter referred to as patent document 1). The digital predistortion method features elimination of the need for complex analog circuitry by implementing a predistorter configuration through utilization of digital signal processing. Linear amplifier realizations have been achieved mostly by use of analog circuits such as a feed-forward amplifier, a negative feed-back amplifier, and so forth. Predistorters have also been formed by analog circuits (for example, H. Girard and K. Feher, “A New Baseband Linearizer for More Efficient Utilization of Earth Station Amplifiers Used for QPSK Transmission,” IEEE J. Select. Areas Commun. SAC-1, No. 1, 1983, hereinafter referred to as non-patent document 1, and Nojima and Okamoto, “Analysis and Compensation of TWT Nonlinearities Based on Complex Power Series Representation,” Journal of the Society of Electronics, Information and Communication Engineers of Japan, B, Vol. J64-B, No 12, Dec. 19812, hereinafter referred to as non-patent document 2). But linearizer technologies using such analog circuits generally require a high accuracy by adjusting technique. Furthermore, the analog circuits need to be simplified for miniaturization and economization of a transmitter containing a modulation circuit.
There has been known a digital predistorter of the type having a look-up table for pre-linearizing the nonlinear characteristics of an amplifier (for example, patent document 1, and L. Sundstorm, M. Faulkner, and M. Johansson, “Quantization Analysis and Design of a Digital Predistortion Linearizer for RF Power Amplifiers,” IEEE Trans., Vech. Tech., Vol. 45, No. 4, pp. 707–719, 1996, 11, hereinafter referred to as non-patent document 3). The digital predistorter with the look-up table updates values set in the look-up table by feeding back thereto the amplifier output signal to keep the levels of distortion components down under a set value. It is well-known in the art that distortion compensation can be achieved by such digital signal processing, and that the amount of compensation is equal to or smaller than approximately 15 dB (for example, Ishikawa, Hase, Kubo, Tozawa, and Hamano, “Development of Adaptive Distortion Compensator for W-CDMA Base Stations,” 2002 Assembly of the Society of Electronics, Information and Communication Engineers of Japan, C-2-31, 2002, 09, hereinafter referred to as non-patent document 4).
In general, the look-up table type predistorter achieves distortion compensation over a frequency range of only 20 MHz or so about the carrier frequency. On this account, in the PDC (Personal Digital Cellular) system in Japan or the like which simultaneously transmits signals of both of 800 MHz and 1.5 GHz bands, for instance, it is impossible to achieve predetermined compensation of the signals of the both frequency bands at the same time. To transmit signals of multiple frequency bands while making distortion compensation of the signals by look-up table type predistortion processing, the look-up table type predistorter needs to be built for each frequency band, and a vector signal modulator, a frequency converter, and a power amplifier are also required for each band—this inevitably causes the defects of bloated device scale, higher power consumption and increased device bulkiness.
Common processing of the look-up table type predistortion for multiple frequency bands would permit simplification of the device configuration, reduction of power consumption and device miniaturization. In the case of performing the look-up table type predistortion processing for multiple frequency bands by use of a single look-up table, however, if the bands are spaced far apart as compared with their bandwidths as in the cases of 800 MHz and 1.5 GHz, the compensation values prestored in the look-up table become uncertain for the signals of the both bands, making it impossible to correctly compensate for distortions occurring in the both bands.